The present invention relates to an automatic overload release and power stoppage device installed in a power tool to automatically cut off power supply upon an overload.
In a motor-driven or pneumatic screw driver, drill, or any of a variety of torque tools, there is generally provided an overload protecting device to protect against overload. The most commonly used overload protecting device is an overload circuit breaker. FIG. 15 illustrates an overload circuit breaker for a power tool according to the prior art. The shaft of the power tool has a through hole into which a link is inserted. As the shaft is depressed, the link to turn on the power switch. Power supply is sent through a triac, then rectified, and then sent to the motor, causing the motor to produce a torsional force. As the workpiece, for example: a screw or nut, is fastened tight, the clutch provides a signal to trigger the circuit breaker, causing it to cut off the power switch of the motor. By means of the operation of a time delay circuit, the circuit breaker is started again to short circuit the two opposite poles of the motor, and therefore the motor is stopped. Sparks are produced each time the motor or the circuit breaker is started. The sparks will damage the carbon brush and the rectifier of the motor easily. This structure of circuit breaker is complicated, and expensive to manufacture. Further, it needs much installation space. During the operation of the circuit breaker, the triac may be falsely triggered by serge. If the triac is triggered falsely as the circuit breaker is electrically connected, a short circuit will occur, the related parts will be burnt out. If a surge absorber is installed, a satisfactory result is still not achievable, and much installation space is needed.